1. Field of the Invention
This invention relates to signal processing.
2. Background Information
Signals carrying perceptual information, such as video and audio signals, are commonly transferred in analog form. Such analog transfer may occur even in cases where the perceptual information was created digitally or obtained from a digital source. For example, personal computers and other video generation devices (e.g. digital versatile disk (DVD) players) typically output analog video signals for display on cathode-ray-tube (CRT) monitors or other devices including CRTs such as television sets.
Unfortunately, the analog signal may become degraded during transfer. For example, environmental electromagnetic interference may produce noise in the signal. Other environmental factors such as temperature may affect timing or other characteristics of circuitry used to process the signal. High frequencies of the signal may also be attenuated by the signal path. Depending upon the particular cause, such degradations may occur in high-frequency and/or in low-frequency regions of the signal, and upon display of the signal they may be manifested by effects such as nonuniformities, random specks, or reductions in resolution or sharpness.
Noise problems may also become worse as the number of pixels in a display grows. For example, a large image display has more pixels than a small one. A phase-locked loop (PLL) may be used to control a clock of an analog-to-digital converter for digitizing the signal for display on a digital display (e.g. a liquid-crystal display). As the number of pixels increases, the PLL must run at a higher speed, which may give rise to increased jitter. Such increased jitter may be visible in the displayed signal as noise.
Noise may also cause the appearance of undesirable artifacts in low-frequency regions of a video signal. Such effects may be visible as variation, as the eye is more sensitive to noise in these regions. Noise of this type may be due to amplitude and/or phase variations in analog signal.
Moreover, the quality of an RGB-out signal may vary greatly. For example, it may be desired to display a signal from a video source whose output signal deviates from the VESA standard.
Automatic gain control may be used to control some noise problems, but at some point such techniques may also adversely affect signal contrast and brightness.